Reusable document technologies allow a print substrate, such as paper, to be printed upon more than one time. Whereas non-reusable document printing has a high environmental impact as a result of disposal of the substrate, reusable paper (for example) can provide economic and environmental advantages for routine printing needs. One method of reusable printing is to bleach the ink printed onto the substrate, such that new ink can be printed on top of the non-visible bleached ink.
Semiconductor metal oxides are among the most commonly used photocatalytic materials for the decomposition of organic molecules. Such semiconductor metal oxides have been used for the photodegradation of a variety of compounds, including surfactants, pesticides, and dyes. In the presence of air, humidity, and light, many semiconductor metal oxides produce reactive species that can initiate the photodegradation of organic molecules such as dyes.
For example, doped and undoped titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles (30-50 nm) have been extensively applied to the photobleaching of dyes in wastewater from textile processing and the photocatalytic removal of other aqueous pollutants. For example, Agostiano et al. have shown that titanium dioxide can be used to bleach textile dye Uniblue A. P. D. Cozzoli, R. Comparelli, E. Fanizza, M. L. Cut, A. Agostiano (2003) Mater. Sci. Eng. C 23 707-713, the text of which is hereby incorporated in its entirety. The generation of reactive species is a surface phenomenon, so the catalytic efficiency of these materials can be improved through the use of nanoparticle-sized materials.
A variety of publications discuss the use of photocatalytic particles in the context of printing. For example, U.S. Patent Application Publication No. 2006/0137841 to Chatani et al. describes a printing paper substrate that is coated with photocatalytically active titanium dioxide particles. The titanium dioxide provides an “air cleaning effect” as well as “good printability” and “good color print quality.” Chatani does not discuss reusing the substrate by utilizing the photocatalytically active particles to bleach an ink.
In Japanese Patent Application Publication No. 2003-073587, there is provided an ink composition that is erasable by light irradiation. Specifically, there is provided an aqueous ink composition comprising a dye and titanium oxide nanoparticles that act as photocatalysts.
However, dye based inks may comprise a wide variety of solvents, not merely aqueous solutions, in order to achieve particular desired compositions or drying properties. Solvent-based inks are popular for numerous printing applications. On the other hand, the ink disclosed by JP 2003-073587 is limited to aqueous solutions, because titanium oxide is not easily disperable in other systems such as organic solutions.
Furthermore, titanium oxide is poorly disperable even in aqueous environments, thus requiring the use of a wetting agent as disclosed in JP 2003-073587. The use of a wetting agent is unlikely to achieve the dispersion quality and stability necessary for demanding print applications, such as ink jet. As a result of poor dispersability, the photocatalytic bleaching effect may be uneven. Such unevenness can result in portions of the printed image remaining visible, thus making the substrate not reusable, and thereby defeating the purpose of using such a system.
Therefore, there exists in the art a need for improved photo-erasable inks.